The present invention relates to waveguide mode converters, and more particularly to a mode converter that converts directly from a circular waveguide mode having no angular dependence to the HE.sub.11 mode.
Waveguides are a form of transmission line used to transmit electromagnetic energy efficiently from one point to another. Waveguide modes are denominated to identify the distribution of the electric and magnetic fields within the waveguide. As indicated in the Electronics Designers' Handbook, 24 Edition (McGraw-Hill 1977) at page 8-36, specific modes are indicated by symbols such as TE.sub.mn and TM.sub.mn. TM indicates that the magnetic field is everywhere transverse to the axis of the transmission line, i.e., the longitudinal axis of the waveguide. TE indicates that the electric field is everywhere transverse to the axis of the waveguide. The subscripts m and n denote the number of full or half period variations of the fields occurring within the waveguide, as explained more fully below.
In addition to the TE and TM modes, an HE.sub.mn mode also exists for circular corrugated waveguides. This mode is described in the literature. See, e.g., Doane, "Propagation and Mode Coupling in Corrugated and SmoothWall Circular Waveguides," Infrared and Millimeter Waves, Vol. 13, Chapter 5, pp. 123-170 (Academic Press, 1985). The HE mode is somewhat similar to the TE mode, except that a different radiation pattern of the electromagnetic energy is obtained when the waveguide terminates. As explained hereinafter, such a radiation pattern offers distinct advantages over other patterns obtained from other modes.
For circular waveguides, the subscript m used with the waveguide symbol denotes the number of full-period variations of the transverse component of field in the angular direction. The subscript n denotes the number of half-period variations of the transverse component of field in the radial direction. A waveguide mode having no angular dependence may thus be either a TE.sub.0n or a TM.sub.0n mode, where n is any integer. The present invention is thus concerned with a waveguide mode converter that converts directly from a TE.sub.0n or a TM.sub.0n mode to the HE.sub.11 mode.
Waveguides with transverse dimensions large compared to wavelength become necessary at millimeter wavelengths in order to reduce loss and to prevent breakdown in high power applications. Such waveguides are called "overmoded" since more than one waveguide mode can propagate. See, Doane, supra A significant problem facing waveguide mode converters of overmoded waveguides is to confine the available energy to the desired modes, and to prevent energy from being coupled to undesired modes. This requirement is frequently referred to as minimizing mode competition or reducing cross-coupling.
The HE.sub.11 mode advantageously radiates a symmetric pencil beam having low side lobes and low cross polarization. This type of radiation has application to, e.g., antenna structures, laser devices, fiber optics, and rocket launching systems. Unfortunately, most sources of microwave energy provide an output mode of transmission other than the HE.sub.11 mode of transmission, such as the TE.sub.01 mode of transmission. Hence, there is a need to convert the transmission mode of the energy source to the HE.sub.11 mode before the advantages of the HE.sub.11 mode can be fully exploited.
Moreover, for high energy applications, such as rocket launching systems (where the high energy microwave signals are used for plasma heating), or sophisticated high power radar systems, the source of the high energy signal is typically a gyrotron, or equivalent device, which cannot always be positioned near the location in the apparatus or system where the HE.sub.11 mode of transmission is required. While the HE.sub.11 mode can be transmitted efficiently (without significant loss) through a corrugated waveguide, the cost of corrugated waveguide per unit length is much higher than the cost of smooth-wall waveguide per unit length. Hence, where the transmission distance is more than just a few meters, the less-costly smooth-wall waveguide becomes the preferred mode of transmission. There is thus a need to: (1) transfer the energy from the source to its destination using a cost-effective smooth-wall waveguide operating in an appropriate mode, such as the TE.sub.01 mode of transmission, and (2) convert the mode of transmission to the HE.sub.11 mode once the energy has been delivered to its desired destination within the system.
There are no conversion methods known at present that convert directly from the optimum transmission mode, e.g., the TE.sub.01 or TM.sub.01 mode, to the desired HE.sub.11 mode. Rather, known conversion systems utilize a two stage method to achieve the desired conversion. See, e.g., Doane, "Mode converters for generating the HE 11 (gaussian-like) mode from TE 01 in a circular waveguide," Int. J. Electronics, Vol. 53, No. 6, pp. 573-585 (1982). That is, a first conversion is made from the optimum TE.sub.01 transmission mode to an intermediate mode; and a second conversion is then made from the intermediate mode to the desired HE.sub.11 mode. The intermediate mode is typically either the TE.sub.11 mode or the TM.sub.11 mode.
In the case where the intermediate mode is the TE.sub.11 mode, a TE.sub.01 to TE.sub.11 converter is used as a first stage. One common embodiment of such a TE.sub.01 to TE.sub.11 converter comprises a specially machined waveguide having periodic radial perturbations. See, Moeller, "Mode converters used in the Doublet III ECH microwave system," Int. J. Electronics, Vol. 53, No. 6, pp. 587-593 (1982). Unfortunately, such a converter has a narrow bandwidth, requires high machining tolerances and must include many periods in its overall length. Another embodiment utilizes periodic perturbations of special shape in order to avoid competition with the TE.sub.12 mode.
Disadvantageously, in both embodiments the ohmic losses of the TE.sub.11 mode limit the permissible length of the converter. Moreover, the second stage, the TE.sub.11 to HE.sub.11 converter, also has a limited bandwidth because it contains an abrupt transition from smooth wall waveguide to corrugated wall waveguide, having one-half wavelength deep corrugations. This abrupt transition necessarily causes a narrow band width.
In the case where the intermediate mode is the TM.sub.11 mode, a first stage of the desired converter comprises a TE.sub.01 to TM.sub.11 converter that includes a smooth-wall circular waveguide that curves or bends a prescribed amount within a plane while accurately maintaining the circularity of the waveguide's bore. Such a first stage requires tight machining tolerances, and is thus expensive and difficult to make. Moreover, the TM.sub.11 mode inherently has substantial ohmic loss associated with its operation, as well as high electric fields present at the waveguide wall. The substantial ohmic loss disadvantageously affects the overall efficiency of the converter, and the high electric field at the waveguide wall makes the waveguide susceptible to breakdown. While a second stage of the desired converter, comprising a TM.sub.11 to HE.sub.11 converter, has a wider bandwidth and a less critical transition from smooth to corrugated waveguide than does its TE.sub.01 to TM.sub.11 counterpart, the inefficiencies of the first stage prevent an efficient overall conversion.
It is thus apparent that a more efficient TE.sub.01 or TM.sub.01 to HE.sub.11 converter is needed, preferably one that contains only low loss modes, has a wide bandwidth, does not suffer from mode competition, and is easily and inexpensively fabricated. The present invention advantageously addresses these and other needs.